Thermal and kinetic analysis of uranium salts

Thermal decomposition of U(C₂O₄)₂·6H₂O was studied using TG method in nitrogen, air, and oxygen atmospheres. The decomposition proceeded in five stages. The first three stages were dehydration reactions and corresponded to removal of four, one, and one mole water, respectively. Anhydrous salt decomposed to oxide products in two stages. The decomposition products in nitrogen atmosphere were different from those in air and oxygen atmospheres. In nitrogen atmosphere UO_1.5(CO₃)_0.5 was the first product and U₂O₅ was the second product, while these in air and oxygen atmospheres were UO(CO₃) and UO₃, respectively. The second decomposition products were not stable and converted to stable oxides (nitrogen: UO₂, air–oxygen: U₃O₈). The kinetics of each reaction was investigated with using Kissinger–Akahira–Sunose and Flynn–Wall–Ozawa methods. These methods were combined with modeling equations for thermodynamic functions, the effective models were investigated and thermodynamic values were calculated.     

Thermal decomposition of [Co(NH3)6]2(C2O4)3·4H2O: II. Identification of the gaseous products

The main goal of the presented work was to verify the previously assumed decomposition stages of [Co(NH₃)₆]₂(C₂O₄)₃·4H₂O (HACOT) [Thermochim. Acta 354 (2000) 45] under different atmospheres (inert, oxidising and reducing). The gaseous products of the decomposition were qualitatively and quantitatively analysed by mass spectrometry (MS) and Fourier-transformed infrared spectroscopy (FT-IR). It was confirmed that the gaseous products of HACOT decomposition under studied atmospheres there were H₂O (stage I) and NH₃, CO₂ (stage II). The main gaseous products in the third stage in argon and hydrogen (20 vol.% H₂/Ar) were CO and CO₂, whereas in air (20 vol.% O₂/Ar) only CO₂ was identified. Under the oxidising as well as reducing atmospheres the influence of secondary reactions on the composition of both, solid and gaseous products was found particularly strong during the third stage of the process. The studies of the multistage decomposition of HACOT, additionally complicated by many secondary reactions, required application of the hyphenated TA-MS or TA-FT-IR techniques combined with the pulse thermal analysis PTA^® allowing quantification of the spectroscopic signals and investigation of gas-solid and gas-gas reactions in situ.     

Thermal Analysis of Cobalt(II) Salts with some Carboxylic Acids

The thermal analysis of CoC₂O₄·2H₂O, Co(HCOO)₂·2H₂O and Co(CH₃COO)₂·4H₂O was carried out with simultaneous TG-DTG-DTA measurements under non-isothermal conditions in air and argon atmospheres. The intermediates and the end products of decomposition were characterised by X-ray diffraction and IR and UV-VIS spectroscopy. The decomposition of the studied compounds occur in several stages. The first stage of dissociation of each compound is dehydration both in air and argon. The next stages differ in air and argon. The final product of the decomposition of each compound in air is Co₃O₄. In argon it is a mixture of Co and CoO for cobalt(II) oxalate and cobalt(II) formate but CoO for cobalt(II) acetate. This revised version was published online in August 2006 with corrections to the Cover Date.     

Synthesis of γ-Fe2O3 by Thermal Decomposition of Ferrous Gluconate Dihydrate

Ferrous gluconate dihydrate (FeC₁₂H₂₂O₁₄⋅2H₂O), was prepared and its thermal decomposition was studied by means of simultaneous thermal analysis, supplemented with a two probe d.c. electrical conductivity measurements under the atmospheres of static air, dynamic air and dynamic nitrogen. Under all the atmospheres final product was found to be α-Fe₂O₃ with FeO, γ-Fe₂O₃, Fe₃O₄ etc. as probable intermediates. γ-Fe₂O₃ was formed under the atmosphere of dynamic air containing water vapour. γ-Fe₂O₃ thus synthesised was characterised for its structure, morphology, thermal and magnetic behaviour. This revised version was published online in August 2006 with corrections to the Cover Date.     

Thermal decomposition of scandium(III) benzenetricarboxylates in air and nitrogen atmospheres

The conditions of thermal decomposition of scandium(III) hemimellitate, trimellitate and trimezinate in air and nitrogen atmospheres have been studied. On heating, the benzene-tricarboxylates of Sc(III) decompose in two stages. First, the hydrated complexes lose crystallization water; heating in air finally yields Sc₂O₃, and heating in a nitrogen atmosphere Sc₂O₃ and C. The dehydration of the complexes is associated with strong endothermic effects. The decomposition of benzenetricarboxylates in air is accompanied by an exothermic effect and in nitrogen by an endothermic effect. The activation energies of the dehydration and decomposition reactions have been calculated for the Sc(III) benzenetricarboxylates.     

Thermal and kinetic analysis of uranium salts

The thermal decomposition kinetics of UO₂C₂O₄·3H₂O were studied by TG method in a flowing nitrogen, air, and oxygen atmospheres. It is found that UO₂C₂O₄·3H₂O decomposes to uranium oxides in four stages in all atmosphere. The first two stages are the same in the whole atmosphere that correspond to dehydration reactions. The last two stages correspond to decomposition reactions. Final decomposition products are determined with X-Ray powder diffraction method. Decomposition mechanisms are different in nitrogen atmosphere from air and oxygen atmosphere. The activation energies of all reactions were calculated by model-free (KAS and FWO) methods. For investigation of reaction models, 13 kinetic model equations were tested and correct models, giving the highest linear regression, lowest standard deviation, and agreement of activation energy value to those obtained from KAS and FWO equations were found. The optimized value of activation energy and Arrhenius factor were calculated with the best model equation. Using these values, thermodynamic functions (??H ^*, ??S ^*, and ??G ^*) were calculated.     

Investigation on Thermal Decomposition of FeS2 and BaO2 Mixtures Part II

The thermal decomposition of FeS₂ and BaO₂ mixtures (mol ratio from 2 to 8) was studied in oxygen containing gas medium using dynamic heating rate. The solid decomposition products have been investigated with X-ray power diffraction and Mössbauer spectrometer. The thermal process has two main stages. In the presence of BaO₂ the mixtures have a lower initial temperature of iron sulfide burning. The same time by the increasing of BaO₂ content in the mixtures the diffusion difficulties are withdrawn in higher temperature ranges. It is proved that as intermediates BaSO₄, nonstoichiometric sulfide, barium ferrites and Fe₂O₃ are formed. The content of many solid phases in the final product is in relationship with the initial ratio of BaO₂ and FeS₂.     

The non-isothermal decomposition of cobalt acetate tetrahydrate

The non-isothermal decomposition of cobalt acetate tetrahydrate was studied up to 500°C by means of TG, DTG, DTA and DSC techniques in different atmospheres of N₂, H₂ and in air. The complete course of the decomposition is described on the basis of six thermal events. Two intermediate compounds (i.e. acetyl cobalt acetate and cobalt acetate hydroxide) were found to participate in the decomposition reaction. IR spectroscopy, mass spectrometry and X-ray diffraction analysis were used to identify the solid products of calcination at different temperatures and in different atmospheres. CoO was identified as the final solid product in N₂, and Co₃O₄ was produced in air. A hydrogen atmosphere, on the other hand, produces cobalt metal. Scanning electron microscopy was used to investigate the solid decomposition products at different stages of the reaction. Identification of the volatile gaseous products (in nitrogen and in oxygen) was performed using gas chromatography. The main products were: acetone, acetic acid, CO₂ and acetaldehyde. The proportions of these products varied with the decomposition temperature and the prevailing atmosphere. Kinetic parameters (e.g.E and lnA) together with thermodynamic functions (e.g. °H, C _p and °S) were calculated for the different decomposition steps. In celebration of the 60^th birthday of Dr. Andrew K. Galwey     

Isothermal decomposition of K2C2O4

The effect of semiconducting metal oxide (CuO and TiO₂) additives on the kinetics of thermal decomposition of potassium oxalate (K₂C₂O₄) to potassium carbonate has been studied at five different temperatures in the range 793–813 K under isothermal conditions by thermogravimetry (TG). The decomposition is enhanced by CuO (p-type) and suppressed by TiO₂ (n-type). The diverse behaviour of K₂C₂O₄ in the presence of different types of oxides in contrast with the like behaviour of K₂C₂O₄ suggests the involvement of different rate determining steps in the decomposition of these solids. The TG data of 2 mass% oxide mixed samples of K₂C₂O₄ were subjected to both model fitting and model-free (isoconversional) kinetic methods of analysis. The model fitting method of analysis shows that the rate law for the decomposition of K₂C₂O₄ (Prout–Tompkins and contracting cylinder models, respectively, for the acceleratory and decay stages) remained unaffected by the additives.     

硝酸钕水合物热分解机理及脱水过程动力学的研究

本文采用TG-DTG法研究了Nd(NO)·nHO(n=6,5,4)的热分解行为,并通过IR对反应物、中间产物和最终产物进行了鉴别。发现中间产物有低水合物、无水盐和碱式盐,最终产物是氧化钕。另外,还进行了上述样品某些脱水过程的动力学研究,借助不同升温速率下的TG-DTG曲线,应用Kissinger法计算了它们脱水的表观活化能值,并利用DSC求出了它们的脱水焓值。     

Investigation of UO4·2NH3·2HF by simultaneous thermogravimetry-differential thermal analysis-mass spectrometry technique

Thermal decomposition of UO₄·2NH₃·2HF was studied under high vacuum and in different gas atmospheres (N₂, O₂, synth. air). Gaseous decomposition products were analyzed and recorded using a quadrupole mass spectrometer.By discussing TG and MS data as well as X-ray analysis of intermediate products an attempt is made, to explain decomposition mechanisms under varied experimental conditions.Thermal decomposition strongly supports the results of X-ray analysis leading to the formula UO₄·2NH₃·2HF.     

Investigations on the formation of rubidium dimolybdate via thermal decomposition of rubidium oxomolybdenum(VI) oxalate

The complex Rb₂[Mo₂O₅(C₂O₄)₂(H₂O)₂] (RMO) was prepared and characterized by means of chemical analysis and IR spectral studies. Its thermal decomposition was studied by using TG and DTA techniques. RMO loses its water between 160 and 200°C, this immediately being followed by the decomposition of anhydrous RMO, which takes place in three stages. The first two stages occur in the temperature ranges 200–220 and 220–255°, to give intermediates with tentative compositions Rb₈[Mo₈O₂₂(C₂O₄)₆] and Rb₈[Mo₈O₂₆(C₂O₄)(CO₃)], respectively, the latter then decomposing in the third stage between 255 and 340° to give the end-product, rubidium dimolybdate (Rb₂Mo₂O₇). Thed spacings for Rb₂Mo₂O₇ are given for 2θ values between 10 and 70°.     

Effects of different additives on the thermal decomposition of ammonium perchlorate

IR spectral investigations on ammonium perchlorate (AP) in the presence of varying amounts of ammonium permanganate (APm) and 2% by weight of different rare earth oxide additives were made over the temperature range 25–290°. Heating and spectral scanning were done simultaneously and the peak intensity was presumed to be proportional to the amount of undecomposed AP. Presence of 10% APm lowered the temperature of AP decomposition from 200 to 110° and increased the rate by several folds. Mixed oxide produced as a result of deflagration of APm are considered catalyzing the process. In the presence of rare earth oxides additives, the NH ₄ ⁺ stretching peak intensity decreased considerably, the extent followed the trend Gd₂O₃ > MnO₂ > Nd₂O₃ > Pr₂O₃ > Dy₂O₃ > Y₂O₃ > La₂O₃ > virgin AP. An electron transfer mechanism is envisaged to explain the results.     

Mechanism and kinetics of inorganic sulphates decomposition

Thermal decomposition of different inorganic sulphates are presented. A number of techniques, but mainly TG and DTA, are used to prove the mechanism and kinetics of CaSO₄, BaSO₄, FeSO₄·xH₂O, Al₂(SO₄)₃·xH₂O under various gas atmospheres. It is shown how the partial pressure of gas components and heating rate may effect the mechanism and kinetic parameters. There are also examples on the effects of some additives and initial treatment on the thermal processes. On the base of the results obtained some recommendations are given concerning the precautions to be taken into account in the thermal decomposition studies and the sulphur recovering.     

Transformations of manganese oxides under different thermal conditions

Thermal decomposition of various synthetic manganese oxides (MnO, Mn₃O₄, Mn₂O₃, MnOOH) and a natural manganese dioxide (MnO₂) from Gabon was studied with the help of termogravimetry in inert, oxidizing and reducing atmospheres. The compounds were characterized by XRD and electrochemical activity was tested by cyclic voltammetry using a carbon paste electrode. The natural manganese dioxide showed the best oxidizing and reducing capacity, confirmed by the lower temperatures of the transitions, the extent of the reactions and electrochemical performance in cyclic voltammograms.     

Effects of alkali metal ions on the thermal decomposition of ammonium heptamolybdate tetrahydrate

The thermal decomposition of pure ammonium heptamolybdate tetrahydrate (AHMT), and doped with Li⁺, Na⁺ and K⁺ ions was investigated using thermogravimetry, differential thermal analysis, infrared and X-ray diffraction techniques. Results obtained revealed that the decomposition of AHMT proceeded in three decomposition stages in which both NH₃ and H₂O were released in all stages. The presence of 0.5 mol % alkali metal ions enhances the formation of the intermediateb (NH₄)₂MO₇O₂₂·2H₂O while the decomposition of this intermediate into MoO₃ is slightly affected in the presence of all dopant concentrations used. The infrared absorption spectra of the thermal products of AHMT treated with 10 mol% alkali metal ions (AMI) at 350°C indicated a reduction of some Mo⁶⁺ ions. By heating of AHMT above 500°C in presence of 5 or 10 mol % of AMI, a solid-solid interaction between alkali metal oxides and MoO₃ giving rise to well crystallized alkali metal molybdates. finally the activation energies accompanied various decomposition stages were calculated.     

Effect of nano-transition metal oxides of Fe,Co and Ni and ferrites of Co and Ni on the multistage thermal decomposition of oxalates of Ce(III)

Semiconducting nano-metal oxides (Fe₃O₄, Co₃O₄, NiO, CoFe₂O₄ and NiFe₂O₄) were synthesized by thermal decomposition of their oxalate precursors. Using DSC technique, effect of nano-metal oxides [5 m m^?1 (%)] on the reaction pathway and mechanism of thermal decomposition of Ce₂ (C₂O₄)₃·10H₂O in flowing atmosphere of N₂ was investigated under linear non-isothermal condition. Performing the kinetic deconvolution method, physico-geometrical kinetic behavior of the two overlapping heat absorbing steps of both lower and higher temperature reactions was illustrated. Nano-Fe₃O₄ promoted the dehydration stages by lowering the Ea value to 35–36 kJ mol^?1. Nano-Co₃O₄, nano-CoFe₂O₄ and nano-NiFe₂O₄ promoted the dehydration as well as decomposition stages of cerium oxalate decahydrate by decreasing the Ea value. Nano-NiO has shown retarding effect on both dehydration and decomposition stages.

     

Ceramic electrolytes based on (Ba1 − x Ca x )(Zr0.9Y0.1)O3 solid solution

The aim of this work was to study the electrical and electrochemical properties of the (Ba_1???x Ca _x )(Zr_0.9Y_0.1)O₃ solid solutions. The powders of different calcium content (x?=?0, 0.05, 0.1, and 1) were prepared by a thermal decomposition of organo-metallic precursors containing ethylenediaminetetraacetate acid. X-ray diffraction analysis showed that a small substitution of calcium for barium caused formation of cubic solid solutions with the decreasing cell parameters. Electrical conductivity measurements were performed by the d.c. four-probe method in controlled gas atmospheres containing Ar, air, H₂, and/or H₂O at temperature from 300 to 800 °C. It was found that the conductivity depended on a chemical composition of the samples and the atmosphere. Overall, the electrical conductivity was higher in wet atmospheres that contained oxygen that was in accordance with the model of a proton transport in perovskite structure which assumed the presence of the oxygen vacancies. The solid solution containing 5 mol% of calcium showed the highest conductivity and the lowest activation energy of conductivity regardless of the atmospheres; this can be attributed to the local changes in the cubic perovskite structure. Test results for CaZr_0.9Y_0.1O₃ used as an electrolyte in solid oxide galvanic cells involving CaCr₂O₄ as a reference electrode are also reported.     

Thermal and kinetic analysis of uranium salts

In this study the thermal decomposition kinetics of uranyl acetate dehydrate [UO₂(CH₃COO)₂·2H₂O] were studied by thermogravimetry method in flowing nitrogen, air, and oxygen atmospheres. Decomposition process involved two stages for completion in all atmosphere conditions. The first stage corresponded to the removal of two?moles of crystal water. The decomposition reaction mechanism of the second stage in nitrogen atmosphere was different from that in air and oxygen atmospheres. Final decomposition products were determined with X-ray powder diffraction method. According to these data, UO₂ is the final product in nitrogen atmosphere, whereas U₃O₈ is the final product in air and oxygen atmospheres. The calculations of activation energies of all reactions were realized by means of model-free and modeling methods. Kissinger?CAkahira?CSunose (KAS) and Flynn?CWall?COzawa (FWO) methods were selected for model-free calculations. For investigation of reaction models, 13 kinetic model equations were tested. The model, which gave the highest linear regression, the lowest standard deviation, and an activation energy value which was close to those obtained from KAS and FWO equations, was selected as the appropriate model. The optimized value of activation energy and Arrhenius factor were calculated using the selected model equation. Using these values, thermodynamic functions (??H*, ??S*, and ??G*) were calculated.     

New data on the mechanism of hydrogen peroxide decomposition in acetic acid in the presence of vanadyl and cobalt (II) acetylacetonates

The process of catalytic hydrogen peroxide decomposition in acetic acid in the presence of vanadyl and cobalt (II) acetylacetonates was studied using modern spectroscopic and kinetic techniques. The formation of intermediates during the catalytic decomposition of hydrogen peroxide in the presence of VO(acac)₂ was observed using UV—Vis and ESR spectroscopy. The decomposition of H₂O₂ occurs both catalytically and via the radical route.